3.1 Misconceptions

Question 1

Explain why amylase produced in the human digestive system does not digest GOS? (3)

Answer 1

1. Active site (only) complementary to starch
   OR
   Active site not complementary. to GOS;
   Ignore ‘hydrolysis’
2. (Due to) tertiary structure;

Question 2

Prebiotics stimulate the growth of healthy bacteria. Suggest how GOS (a polysaccharide of galactose) can work as a prebiotic? (3)

Answer 2

1. Provides galactose/sugar/monosaccharide;
2. (Bacteria use the galactose/sugar) for respiration;
3. (Bacteria use the galactose/sugar) for binary fission;
   Reject ‘glucose’ for galactose/sugar once only
   Accept (cell) division /replication/reproduction for binary fission
   Reject mitosis;

Question 3

Explain why phospholipids can form a bilayer but triglycerides cannot. (3)

Answer 3

1. Phospholipid both hydrophobic and hydrophilic
   OR
   Phospholipid polar
   OR
   Phosphate group is charged;
2. Triglycerides only hydrophobic
   OR
   Fatty acid/triglyceride is non-polar;
   Accept ‘Triglycerides not hydrophilic’
3. Hydrophilic/phosphate group attracts water (to either side of bilayer);
   Accept ‘faces water’ for ‘attracts water’
   Ignore ‘fatty acids repel water’

Question 4

What term is used to describe the different structures of α-glucose & β-glucose? (1)

Answer 4

Isomer

Question 5

Describe how results from the colorimeter can identify the fruit juice containing the higher sugar content. (1)

Answer 5

High(er) absorbance (has more sugar)
OR
Low(er) transmission (has more sugar);
Accept a description of absorbance or transmission

Question 6

Suggest why the water potential of apple juice decreases when apples are stored. (2)

Answer 6

1. Starch hydrolysed;
2. Maltose is soluble (so reduces water potential)
   OR
   Starch is insoluble;
   Accept glucose for maltose
   Ignore sugar

Question 7

Describe how a quaternary protein is formed from its monomers. (5)

Answer 7

1. Amino acids joined by peptide bond (s);
2. (By) condensation reaction(s);
3. Secondary structure is formed by hydrogen bonding;
   Accept alpha helix OR &-pleated sheet for ‘secondary structure’
4. Tertiary structure formed by interactions (between R groups);
   Accept 3’ for tertiary
5. Quaternary structure contains > 1 polypeptide
   OR
   Quaternary structure formed by interactions/bonds between polypeptides;
   4 and 5 Accept for ‘interactions’, hydrogen bonds OR disulfide bridges OR ionic bonds OR hydrophobic OR hydrophilic interactions
   Ignore peptide

Question 8

Compare and contrast the structure of starch & the structure of cellulose.

Answer 8

1. Both polysaccharides;
   OR
   Both are glucose polymers
   OR
   Both are made of glucose monomers;
2. Both contain glycosidic bonds (between monomers);
3. Both contain carbon, hydrogen and oxygen/C, H and 0;
4. Starch made of a-glucose and cellulose made of B-glucose;
5. Starch (molecule) is helical/coiled and cellulose (molecule) is straight;
6. Starch (molecule) is branched and cellulose is not/unbranched;
7. Cellulose has (micro/macro) fibrils and starch does not;
   Must include 1, 2 OR 3 to achieve 6 marks
   All statements must be clearly comparative or linked by the candidate, not inferred from separate statements
   Additional mark point
   Starch has 1-6 glycosidic bonds and cellulose does not
   OR
   Starch contains two types of molecule and cellulose contains one type of molecule
   OR
   Starch is amylose and amylopectin and cellulose is one type of molecule